Hard material sintered compact with a nickel- and cobalt-free, nitrogenous steel as binder of the hard phase

ABSTRACT

Hard sintered moldings having a nickel- and cobalt-free, nitrogen-containing steel as a binder of the hard phase, processes for the powder metallurgical production of these hard sintered moldings, in particular by powder injection molding, and powder injection molding materials for the production of these hard sintered moldings by powder injection molding.

The present invention relates to hard sintered moldings and feedstocksand processes for their preparation.

For the purposes of this invention, hard sintered moldings are definedas sintered materials which consist of a hard phase and a metallic phaseas a binder of the hard phase. Hard sintered moldings, feedstocks andprocesses for their preparation are well known. Hard sintered moldingsare generally very hard and have a high melting point but are alsoresistant to thermal shocks and therefore constitute a valuable group ofmaterials. They are processed, for example, to combustion chamberlinings or nozzle linings, cutting, drilling, milling, grinding,breaking, digging or press tools, sealing rings or bearing rings,welding electrodes, thread guides or the like. Among the known hardsintered moldings, particularly desirable materials are those whose hardfraction consists of hard ceramic materials, for example oxides,nitrides or carbides. Most frequently used hard materials are tantalumcarbide and tungsten carbide. The metallic binder to be chosen is ametal which can be readily processed to give the hard sintered molding,does not impair the required properties of the material and binds thehard phase in a suitable manner. By far the most frequently used metalsindustrially are nickel and cobalt, but occasionally other metals whichfulfill the required properties are also used. For example, JP-A 63-317601 discloses the use of a cobalt-nickel alloy as a metallic binder.U.S. Pat. No. 3,964,878 describes hard sintered moldings with metalcarbides whose metallic binder consists of the metal also contained inthe carbide and additionally from 0.5 to 1.5% by weight of iron, copperor nickel. EP-A 169 292 and FR-A 1 475 069 describe hard sinteredmoldings having a metallic binder comprising iron, nickel and/or cobalt,the metallic binder of the hard sintered moldings disclosed in EP-A 365506 additionally contains a special high-speed steel, and the metallicbinder of the hard sintered moldings disclosed in JP-A 58-031 059contains iron, nickel, cobalt and/or molybdenum. U.S. Pat. No. 4,308,059describes a hard sintered molding bound by means of ruthenium. EP-A 46209 discloses a hard sintered molding having steel as metallic binder.U.S. Pat. No. 3,368,882 discloses hard sintered moldings comprising25-80% by volume of hard carbide such as WC or TaC and a binder matrixcomprising a nitratable steel which is nitrated 45 on the surface of thesintered molding during preparation of the latter. FR-A-2 058 845describes a hard sintered molding comprising 15-85% by volume of hardcarbide which is dispersed in an austenitic steel matrix.

Hard sintered moldings additionally often have color properties whichlead to an attractive external appearance of the workpieces producedtherefrom and are therefore used not only as material for purelyfunctional components but also as material in decorative applications,for example in watch cases, jewelry, writing implements or the like. Anexample of a known hard sintered molding is given, for example, in JP-A48-018 109, which discloses hard sintered moldings consisting of TaC anda nickel-, molybdenum- and chromium-containing metallic binder andhaving a gold-like surface and their use in watch cases.

Usually, hard sintered moldings are produced by a powder metallurgicalmethod. For this purpose, a mixture of the hard material powder and ametallic powder is introduced into a mold, generally compressed and thensintered, the metal powder and hard material powder combining to givethe hard sintered molding. The sintered molding as such can then befurther processed, for example aftertreated by a shaping procedure, orused, but it may also be milled and applied in the form of powdered hardsintered molding as surface layer on a workpiece. For example, DE-A 4037 480 describes the production of a sintered body comprising tungstencarbide, titanium carbide, tantalum carbide or niobium carbide andcobalt as metallic binder.

A substantial disadvantage of simple powder metallurgical shapingprocesses, for example pressing in a mold, is that only moldings havinga comparatively simple external shape can be produced thereby. Anotherknown powder metallurgical process, which is particularly suitable forthe production of sintered moldings having a complex geometry, is powderinjection molding. For this purpose, a sinterable powder is mixed with athermoplastic, which in powder injection molding technology is usuallyalso referred to as a binder (but must not be confused with the metallicphase referred to in the technology of hard sintered moldings as thebinder of the hard material), and, if required, further assistants, sothat overall a thermoplastic injection molding material (feedstock) isformed. This is injection molded in a mold by the injection moldingtechnology known from the processing of thermoplastics, thethermoplastic powder injection molding binder is then removed from theinjection molded body (green compact), and the body (brown compact)freed from this binder is sintered to give the finished sinteredmolding. The main problem in this process is the binder removal, whichis usually carried out thermally by pyrolysis of the thermoplastic,cracks frequently forming in the workpiece. A thermoplastic which iscatalytically removable at low temperatures is therefore advantageouslyused. EP-A 413 231 describes such a catalytic binder removal process,EP-A 465 940 and EP-A 446 708 disclose feedstocks for the production ofmetallic moldings, and EP-A 444 475 discloses a feedstock for theproduction of ceramic moldings. Furthermore, EP-A 443 048 discloses theproduction of hard sintered moldings by a powder metallurgical method,and EP-A 800 882 describes an improved process for the preparation offeedstocks for hard sintered moldings.

U.S. Pat. No. 5,714,115 discloses a special nickel-free austenitic steelalloy comprising not more than 0.3% by weight of carbon, from 2 to 26%by weight of manganese, from 11 to 24% by weight of chromium, from 2.5to 10% by weight of molybdenum and not more than 8% by weight oftungsten, whose austenitic structure is stabilized by from 0.55 to 1.2%by weight of nitrogen. This alloy is used for workpieces which are incontact, or can come into contact, with the human body, in order toavoid the allergic reactions to nickel or cobalt, which have recentlyincreasingly given rise to concerns. W.-F. Bahre, P. J. Uggowitzer andM. O. Speidel: “Competitive Advantages by Near-Net-Shape-Manufacturing”(Editor H.-D. Kunze), Deutsche Gesellschaft für Metallurgie, Frankfurt,1997 (ISBN 3-88355-246-1) and H. Wohlfromm, M. Blömacher, D. Weinand,E.-M. Langer and M. Schwarz: “Novel Materials in Metal InjectionMolding”, Proceedings of PIM-97 - 1st European Symposium on PowderInjection Moulding, Munich Trade Fair Centre, Munich, Germany, Oct.15-16, 1997, European Powder Metallurgy Association 1997, (ISBN1-899072-05-5), describe powder injection molding processes for theproduction of nickel-free nitrogen-containing steels with nitridingduring the sintering process.

In spite of the well developed prior art, it is an object of the presentinvention to provide novel hard sintered moldings having improved ornovel properties and broader or novel fields of use, in view of theimportance of the class of materials.

We have found that this object is achieved by hard sintered moldingshaving a nickel- and cobalt-free, nitrogen-containing steel as a binderof the hard phase. We have also found a process and feedstocks for theproduction of the novel hard sintered moldings.

The novel sintered moldings have excellent mechanical, thermal andmagnetic properties. They are hard, have a high melting point, arehighly resistant to thermal shocks, are non-magnetic in preferredembodiments and also cause no nickel or cobalt allergies. Moreover, theyexhibit no giant grain growth during the sintering and can be veryreadily polished. They can be produced by the novel process in a simplemanner; in particular, in the production of the novel hard sinteredmoldings, only a comparatively low sintering temperature is requiredcompared with the use of nickel or cobalt binders.

The novel hard sintered moldings contain at least 50, preferably atleast 70, particularly preferably at least 80, % by weight of hardmaterial. They furthermore contain not more than 99, preferably not morethan 97, particularly preferably not more than 95, % by weight of hardmaterial. Accordingly, the novel hard sintered moldings contain at least1, preferably at least 3, particularly preferably at least 5 and notmore than 50, preferably not more than 30, particularly preferably notmore than 20, % by weight of metallic binder.

The metallic binder of the novel hard sintered moldings, its precursoror its components and the hard material are used in the form of finepowders. The mean particle sizes used are usually less than 100,preferably less than 50, particularly preferably less than 20,micrometers and in general approx. 0.1 micrometer. Such powders arecommercially available or can be prepared in any known manner, forexample by precipitation and calcination, or milling, and the metallicpowders can be prepared in particular by water or gas atomization.

Novel hard sintered moldings contain a hard material. All known ceramicsubstances or hard metals used to date as hard materials in known hardsintered moldings may be used, individually or in the form of a mixture,as hard material, for example the oxides, such as alumina, cadmiumoxide, chromium oxide, magnesium oxide, silica, thorium oxide, uraniumoxide and/or zirconium oxide, the carbides, such as boron carbide,zirconium carbide, chromium carbide, silicon carbide, tantalum carbide,titanium carbide, niobium carbide and/or tungsten carbide, the borides,such as chromium boride, titanium boride and/or zirconium boride, thesilicides, such as molybdenum silicide, and/or the nitrides, such assilicon nitride, titanium nitride and/or zirconium nitride, and/or mixedphases, such as carbonitrides, oxycarbides and/or sialons. The use oftantalum carbide, tungsten carbide, niobium carbide, titanium nitrideand/or zirconium nitride is preferred and the use of tantalum carbideand/or tungsten carbide is particularly preferred. These hard materialsare known and are conventional commercial products.

The metallic binder of the novel hard sintered moldings is a nickel- andcobalt-free, nitrogen-containing steel. Freedom from nickel and/orcobalt is to be understood as meaning the absence of intentionally addedamounts of these elements. The permissible upper limit for nickel and/orcobalt in the metallic binder of the novel hard sintered moldings is ingeneral 0.5, preferably 0.3, particularly preferably 0.05, % by weight.These contents are normally below the usual limits for the release ofnickel ions and/or cobalt ions during the use of the workpiece on or inthe human body (as a watch, earpiece, implant, etc.). Very particularlypreferably, the metallic binder contains nickel and/or cobaltexclusively as unavoidable impurities. The steel used as metallic bindercontains nitrogen, preferably in an amount of at least 0.3% by weightand not more than 2% by weight.

The metallic binder is preferably a nonferromagnetic and in particularan austenitic steel. Austenitic steels are known to be those in which aface centered cubic lattice of the iron atoms is present. In theiron/carbon system, the austenite structure is a high-temperaturemodification which is stabilized by specific added alloys at lowtemperatures. Further added alloys impart toughness, corrosionresistance, hardness or other respective desired properties to theaustenitic steels, depending on the requirements. The production,processing and properties of austenitic steels are well known to aperson skilled in the art in the area of engineering materials.

In a particularly preferred form, the metallic binder is an austeniticiron alloy which contains not more than 0.5% by weight of carbon, from 2to 26% by weight of manganese, from 11 to 24% by weight of chromium,from 2.5 to 10% by weight of molybdenum, not more than 8% by weight oftungsten and from 0.55 to 1.2% by weight of nitrogen. Preferably, inaddition to said elements, it contains no further impurities apart fromunavoidable ones. Examples of impurities which can usually be toleratedin the novel hard sintered moldings are up to 0.5% by weight of nickeland/or cobalt, up to 2% by weight of silicon, up to 0.2% by weight ofsulfur, up to 5% by weight of bismuth and up to 5% by weight of copper.

The very particularly preferred metallic binder of the novel hardsintered moldings is austenitic and contains not more than 0.3,preferably not more than 0.1, % by weight of carbon. It contains atleast 2, preferably at least 6, % by weight and not more than 26,preferably not more than 20, % by weight of manganese. It contains atleast 11% by weight and not more than 24, preferably not more than 20, %by weight of chromium. It furthermore contains at least 2.5% by weightand not more than 10, preferably not more than 6, % by weight ofmolybdenum. If particularly high corrosion stability is required, themetallic binder of the novel hard sintered moldings contains tungsten inan amount of not more than 8, preferably not more than 6, % by weight.It furthermore contains at least 0.55, preferably at least 0.7, % byweight and not more than 1.2, preferably not more than 1.1, % by weightof nitrogen. This metallic binder also contains iron over and above saidelements, and the total remainder to 100% by weight, with the exceptionof impurities, is preferably iron.

Such alloys are known to a person skilled in the art, commerciallyavailable or preparable in a simple manner by known metallurgicalprocesses. Since the nitrogen content of these alloys above from 0.8 to0.9% by weight is higher than the nitrogen solubility in the moltenalloy, the alloy must be melted under superatmospheric nitrogenpressure, which is possible, for example, by the electroslag remeltingprocess under pressure. It is just as possible to introduce the nitrogencontent into the metallic binder of the otherwise finished sinteredmolding in a nitriding step by heat treatment in a nitrogen-containingfurnace atmosphere. However, the nitrogen content is preferablyestablished by nitriding during the sintering or immediately before orafter it, without intermediate removal of the sintered molding from thesinter furnace or cooling below the sinter temperature or the nitridingtemperature. Such sinter and nitriding processes are known to a personskilled in the art.

In the case of subsequent nitriding, the corresponding nitrogen-freealloy or an alloy having relatively low nitrogen content should be usedas precursor of the actual metallic binder, said alloy then beingconverted into the metallic binder of the novel hard sintered molding inthe course of the nitriding process. These alloys, too, are commerciallyavailable or can be obtained by smelting in a known manner. In the caseof the preferred austenitic binders, the corresponding nitrogen-freeprecursor is a ferritic steel which is converted into an austeniticsteel by the nitriding.

It is also possible to prepare the metallic binder or its nitrogen-freeprecursor by the master alloy technique known to a person skilled in theart, from a master alloy or a plurality of master alloys, whichessentially contains or contain the elements other than iron and, ifrequired, also iron, and pure iron, so that the novel metallic binderdoes not form until during the sinter and/or nitriding process as aresult of diffusion of the alloy elements, possibly including thenitrogen.

The novel hard sintered moldings are produced by a powder metallurgicalmethod. For this purpose, the hard material and the binder or itsprecursor are mixed in powder form and, by means of a shaping tool,converted into a shape which is as close as possible to its finaldesired geometric shape, in order to avoid any expensive reworking ofthe finished hard sintered molding. The shaping step is carried out bymeans of a conventional shaping tool, for example a press. Duringsintering, shrinkage of the workpieces is known to occur and is usuallycompensated by correspondingly larger dimensioning of the moldings priorto sintering. Thereafter, the molding is sintered in a sinter furnace togive the hard sintered molding and, if a nitrogen-free precursor of themetallic binder or one having a relatively low nitrogen content wasused, the desired nitrogen content is established by nitriding.

That composition of the furnace atmosphere which is optimum for thesintering and, if required, for the nitriding and the operatingtemperature program depend on the exact chemical composition of themetallic binder used or its precursor, in particular itsnitrogen-dissolving capacity, on the desired nitrogen content of themetallic binder and on the particle size of the powders used. Ingeneral, both the increase in the nitrogen partial pressure in thefurnace atmosphere and the reduction in the temperature are directlycorrelated with higher nitrogen contents in the metallic binder.However, since a reduction in the temperature not only causes the sinterprocess itself to slow down but also decreases the rate of diffusion ofthe nitrogen in the metallic binder of the hard sintered molding, thesinter and/or nitriding process takes corresponding longer at lowertemperature. The combination of furnace atmosphere, in particular thenitrogen partial pressure, temperature and duration of sintering and/ornitriding which is optimum for achieving a specific desired nitrogencontent in a homogeneous, dense sintered molding can be readilydetermined from case to case on the basis of a few routine experiments.Such sinter processes have been described for sintered moldingscomprising the steel used in a particularly preferred form as metallicbinder, without a hard phase, for example in the publications by Bahreet al. and Wohlfromm et al. These two publications are herebyincorporated by reference. The properties of the steel do not change asa result of the presence of the hard phase, so that the measuresdescribed there have the same effects in the novel process.

Usually, nitrogen partial pressures in the furnace atmosphere of atleast 0.1, preferably at least 0.25, bar are used. This nitrogen partialpressure is in general not more than 2, preferably not more than 1, bar.The furnace atmosphere may consist of pure nitrogen or may also containinert gases, such as argon, and/or reactive gases, such as hydrogen. Ingeneral, it is advantageous to use a mixture of nitrogen and hydrogen asfurnace atmosphere in order to remove possibly troublesome oxidicimpurities from the metals. The hydrogen content, if present, is ingeneral at least 5, preferably at least 15, % by volume and in generalnot more than 50, preferably not more than 30, % by volume. If desired,this furnace atmosphere may additionally contain inert gases, forexample argon. The furnace atmosphere should preferably be substantiallydry, and in general a dew point of −40° C. is sufficient for thispurpose.

The (absolute) pressure in the sinter and/or nitriding furnace isusually at least 100, preferably at least 250, mbar. In general, it isfurthermore not more than 2.5, preferably not more than 2, bar.Particularly preferably, atmospheric pressure is employed.

The sintering and/or nitriding temperature is in general at least 1000°C., preferably at least 1050° C., particularly preferably at least 1100°C. It is furthermore generally not more than 1450° C., preferably notmore than 1400° C., particularly preferably not more than 1350° C. Thetemperature can be varied during the sinter and/or nitriding process,for example in order to completely or substantially dense-sinter theworkpiece only at a higher temperature and then to establish the desirednitrogen content at a lower temperature.

The optimum heat-up rates are readily determined by a few routineexperiments and are usually at least 1, preferably at least 2,particularly preferably at least 3, ° C. per minute. For economicreasons, a very high heat-up rate is generally desirable in order toavoid an adverse effect on the quality of the sintering and/ornitriding, but it will generally be necessary to establish a heat-uprate below 20° C. per minute. During heating up to the sintering and/ornitriding temperature, it is advantageous in certain circumstances tomaintain a waiting time at a temperature which is below the sinteringand/or nitriding temperature, for example to maintain a temperature offrom 500 to 700° C., for example 600° C., over a period of from 30minutes to 2 hours, for example 1 hour.

The duration of sintering and/or nitriding, i.e. the holding time atsintering and/or nitriding temperature, is generally established so thatthe sintered moldings are both sufficiently densely sintered andsufficiently homogeneously nitrided. At customary sintering and/ornitriding temperatures, nitrogen partial pressures and molding sizes,the duration of sintering and/or nitriding is in general at least 30,preferably at least 60, minutes. This duration of the sinter and/ornitriding process plays a role in determining the production rate, andthe sintering and/or nitriding are therefore preferably carried out sothat the sinter and/or nitriding process does not last for anunsatisfactorily long time from the economic point of view. In general,the sinter and nitriding process (without the heat-up and coolingphases) can be terminated after not more than 10 hours.

The sinter and/or nitriding process is terminated by cooling thesintered moldings. Depending on the composition of the binder, aspecific cooling method may be required, for example very rapid cooling,in order to obtain high-temperature phases or to prevent the separationof the components of the steel. In general, it is also desirable foreconomic reasons to cool as rapidly as possible in order to achieve ahigh production rate. The upper limit of the cooling rate is reachedwhen sintered moldings having defects caused by excessively rapidcooling, such as splitting, cracking or deformation, occur to an extentwhich is unsatisfactorily high in economic terms. The optimum coolingrate is accordingly readily determined in a few routine experiments. Ingeneral, and in particular in the case of the preferred compositions ofthe metallic binder, it is advisable to use cooling rates of at least100, preferably at least 200, ° C. per minute. The sintered molding can,for example, be quenched in cold water or oil.

Following the sintering and/or nitriding, any desired aftertreatment,for example solution heat treatment and quenching in water or oil or hotisostatic pressing of the sintered moldings, can be carried out. Thesintered moldings are preferably subjected to the solution heattreatment by subjecting them to a heat treatment for at least 5,preferably at least 10, minutes and not more than 2 hours, preferablynot more than 1 hour, at a temperature of at least 1000° C., preferablyat least 1100° C., and not more than 1250° C., preferably not more than1200° C., under inert gas, for example under nitrogen and/or argon, andthen quenching them, for example in cold water.

Preferably, and in particular for the production of workpieces having acomplicated geometric shape, the novel hard sintered moldings areproduced by the powder injection molding process. This differs in theprocedure from conventional powder metallurgical processes, such aspressing and sintering, in the manner of the shaping and a resultingadditional step for removing the thermoplastic powder injection moldingbinder used for the shaping. However, the statements made above areapplicable to sintering and nitriding.

To carry out this process, the hard material powder and metal powder aremixed with a thermoplastic, nonmetallic material as a powder injectionmolding binder and the powder injection molding material is thusprepared. Suitable thermoplastics for the preparation of injectionmolding materials are known. In general, thermoplastics are used, forexample, polyolefins, such as polyethylene or polypropylene, orpolyethers, such as polyethylene oxide (polyethylene glycol). The use ofthose thermoplastics which can be removed from the green compactcatalytically at comparatively low temperature is preferred. Apolyacetal plastic is preferably used as the base of the thermoplastic,and polyoxymethylene (POM, paraformaldehyde, paraldehyde) isparticularly preferably used. Assistants for improving the processingproperties of the injection molding material, for example dispersants,are, if desired, also mixed with said injection molding material.Comparable thermoplastic materials and processes for their preparationand processing by injection molding and catalytic binder removal areknown and are described, for example, in EP-A 413 231, EP-A 465 940,EP-A 446 708, EP-A 444 475 and EP-A 800 882, which are herebyincorporated by reference. The metallic or ceramic powder stated therein each case should be replaced accordingly by a powder mixturecomprising the hard material and the metallic binder or its precursor.

A preferred novel injection molding material consists of:

a) from 40 to 65% by volume of a mixture of

a1) from 50 to 99% by weight of a hard material in powder form having amean particle size of at least 0.1 micrometer and not more than 100micrometers and

a2) from 1 to 50% by weight of a nickel- and cobalt-free,nitrogen-containing steel or of a precursor of such a steel in powderform having a mean particle size of at least 0.1 micrometer and not morethan 100 micrometers;

b) from 35 to 60% by volume of a mixture of

b1) from 70 to 90% by weight of a polyoxymethylene homo- or copolymercomprising up to 10 mol% of comonomer units and

b2) from 10 to 30% by weight of a polyoxymethylene copolymer having acomonomer content of from 20 to 99 mol% of poly-1,3-dioxolane,poly-1,3-dioxane or poly-1,3-dioxepan, or of a polymer homogeneouslydissolved in b1) or dispersed with a mean particle size of less than 1micrometer in b1), or mixtures thereof,

 as a thermoplastic powder injection molding binder of the powdermixture a) and

c) from 0 to 5% by volume of a dispersant.

The known ceramic substances or hard metals used in known hard sinteredmoldings are used, individually or in the form of a mixture, as hardmaterial a1), for example oxides, such as alumina, cadmium oxide,chromium oxide, magnesium oxide, silica, thorium oxide, uranium oxideand/or zirconium oxide, carbides, such as boron carbide, zirconiumcarbide, chromium carbide, silicon carbide, tantalum carbide, titaniumcarbide, niobium carbide and/or tunsten carbide, borides, such aschromium boride, titanium boride and/or zirconium boride, silicides,such as molybdenum silicide, and/or nitrides, such as silicon nitride,titanium nitride and/or zirconium nitride, and/or mixed phases, such ascarbonitrides, oxycarbides and/or sialons. The hard material ispreferably tantalum carbide, tungsten carbide, niobium carbide, titaniumnitride and/or zirconium nitride, particularly preferably tantalumcarbide and/or tungsten carbide.

An iron alloy which contains not more than 0.5% by weight of carbon,from 2 to 26% by weight of manganese, from 11 to 24% by weight ofchromium, from 2.5 to 10% by weight of molybdenum and not more than 8%by weight of tungsten is preferably used as the nickel- and cobalt-free,nitrogen-containing steel or precursor of such a steel a2).

An iron alloy which contains not more than 0.3, advantageously not morethan 0.1, % by weight of carbon, at least 2, preferably at least 6, % byweight and not more than 26, advantageously not more than 20, % byweight of manganese, at least 11% by weight and not more than 24,advantageously not more than 20, % by weight of chromium and furthermoreat least 2.5% by weight and not more than 10, advantageously not morethan 6, % by weight of molybdenum is particularly preferably used ascomponent a2). If particularly high corrosion stability of the hardsintered moldings finally to be produced from the injection moldingmaterial is required, component a2) additionally contains tungsten in anamount of not more than 8, preferably not more than 6, % by weight.

The component a2) also contains iron over and above the stated elements.A certain content of impurities in a2) over and above 10 the level ofunavoidable impurities is generally tolerable depending on the use ofthe hard sintered moldings. Examples of impurities which can usually betolerated are up to 0.5% by weight of nickel and/or cobalt, up to 2% byweight of silicon, up to 0.2% by weight of sulfur, up to 5% by weight ofbismuth and up to 5% by weight of copper. However, the total remainderof component a2) to 100% by weight is preferably iron, apart fromunavoidable impurities.

However, it is not necessary to use homogeneous alloys as component a2);rather, the alloy elements may also be present as a mixture of differentalloys and/or pure elements, from which an alloy having the desiredgross composition forms by diffusion in the sinter process according tothe master alloy technique. For example, component a2) may be a mixtureof pure iron powder and an alloy powder which contains the other alloyelements and, if desired, also iron.

The hard material (component a1)) is preferably present in a) in anamount of at least 70, particularly preferably at least 80, % by weight.Its amount is furthermore preferably not more than 97, particularlypreferably not more than 95, % by weight. The metallic binder or itsprecursor (component a2)) is accordingly present in a) preferably in anamount of at least 3, particularly preferably at least 5, % by weightand preferably not more than 30, particularly preferably not more than20, % by weight. The mean particle sizes of hard material a1) andmetallic powder a2) are preferably not more than 50, particularlypreferably not more than 20, micrometers.

The polyoxymethylenehomo- and copolymers used as components b1) and b2)and the polymers optionally used as component b2) and homogeneouslydissolved or dispersed in component b1) are known and are described, forexample as component B1) and B2), respectively, in EP-A 444 475. Thehomopolymers are usually prepared by polymerization (generally catalyzedpolymerization) of formaldehyde or trioxane. For the preparation ofpolyoxymethylene copolymers, a cyclic ether or a plurality of cyclicethers is or are conveniently used as comonomer(s) together withformaldehyde and/or trioxane in the polymerization, so that thepolyoxymethylene chain with its sequence of (—OCH₂) units is interruptedby units in which more than one carbon atom is arranged between twooxygen atoms. Examples of cyclic ethers suitable as comonomers areethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,3-dioxane,1,3-dioxolane, dioxepan, linear oligo- and polyformals, such aspolydioxolane or polydioxepan, and oxymethylene terpolymers. A polymermay also be used as component b2), for example an aliphaticpolyurethane, aliphatic uncrosslinked polyepoxides, poly(C₂-C₅)alkyleneoxides, aliphatic polyamides, polyacrylates, polyolefins and mixturesthereof.

The components b1) and b2) may be identical except for a differentcontent of comonomer(s).

Component c) is a dispersant. Dispersants are widely used and are knownto a person skilled in the art. In general, it is possible to use anydispersant which leads to an improvement in the homogeneity of theinjection molding material. Preferred dispersants are oligomericpolyethylene oxide having an average molecular weight of from 200 to400, stearic acid, hydroxystearic acid, fatty alcohols, fatty alcoholsulfonates and block copolymers of ethylene oxide and propylene oxide. Amixture of different substances having dispersing properties may also beused as the dispersant.

The molding of the novel injection molding materials is effected in aconventional manner using customary injection molding machines. Themoldings are freed from the thermoplastic powder injection moldingbinder in a conventional manner, for example by pyrolysis. The powderinjection molding binder is removed from the preferred novel injectionmolding material preferably catalytically by subjecting the greencompacts in a known manner to a heat treatment with an atmospherecontaining a gaseous acid. This atmosphere is produced by vaporizing anacid with sufficient vapor pressure, conveniently by passing a carriergas, in particular nitrogen, through a storage vessel containing anacid, advantageously nitric acid, and then passing the acid-containinggas into the oven for binder removal. The optimum acid concentration inthe oven for binder removal is dependent on the specific material and onthe dimensions of the workpiece and is determined from case to case byroutine experiments. In general, the treatment in such an atmosphere atfrom 20 to 180° C. over a period of from 10 minutes to 24 hours issufficient for binder removal. After the binder removal, any residues ofthe thermoplastic powder injection molding binder and/or of theassistants still present are pyrolysed by heating up to sinteringtemperature and thus completely removed.

The injection moldings freed from binder, as in the case of moldingsoriginating from other shaping processes, for example pressing, are thensintered and then, if required, nitrided.

EXAMPLES Example 1

3600 g of tantalum carbide powder (mean particle diameter 0.9micrometer), 400 g of a ferritic alloy powder comprising 17% by weightof chromium, 3.4% by weight of molybdenum, and 12.1% by weight ofmanganese, the remainder being iron and unavoidable impurities, inparticular 0.035% by weight of nickel and 0.6% by weight of silicon,also being present (mean particle size 8 micrometers), 64 g ofpolyethylene glycol having an average molecular weight of 800 g/mol, 43g of polybutanediol formal having an average molecular weight of about30,000 glmol and 312 g of polyoxymethylene containing 2% by weight ofbutanediol formal were initially taken in a heatable kneader, melted byheating to 175° C. and homogenized for one hour by kneading. Thereafter,the mixture was cooled and granulated. The granules were injectionmolded to give moldings which were then freed from binder catalyticallyat 120° C. in a nitrogen atmosphere containing nitric acid. Thereafter,the moldings were sintered in a sinter furnace for one hour at 1350° C.and then for 5 hours at 1280° C. in a furnace atmosphere comprising 75%by volume of nitrogen and 25% by volume of hydrogen. The sinteredmoldings were then still very weakly magnetic and were renderedcompletely nonmagnetic by a subsequent heat treatment for 10 minutes at1150° C. under nitrogen followed by quenching in water. The density ofthe pale gold sintered moldings was 13.2 g/ml (theoretical density 13.3g/ml).

The sintered moldings had a Vickers hardness 0.5 of 1400 mm, afour-point flexible strength according to DIN EN 843 (in the as firedstate) of 774 MPa and a cracking resistance K_(lc) according to DIN51109 of 12 MPa(m)^(0.5). The parts were ground and polished. Themicrostructure was homogeneous and showed no crystals having a diameterabove 5 micrometers.

Comparative Example 1

3600 g of tantalum carbide powder (mean particle diameter 0.9micrometer), 400 g of nickel powder (mean particle size less than 10micrometers), 56 g of polyethylene glycol having an average molecularweight of about 800 g/mol, 41 g of polybutanediol formal having anaverage molecular weight of about 30,000 g/mol and 303 g ofpolyoxymethylene containing 2% by weight of butanediol formal wereinitially taken in a heatable kneader, melted by heating to 175° C. andhomogenized for one hour by kneading. The mixture was then cooled andgranulated. The granules were injection molded to give moldings fromwhich the binder was then removed catalytically at 120° C. in a nitrogenatmosphere containing nitric acid. The moldings were then sintered in asinter furnace for one hour at 1500° C. under nitrogen. The density ofthe sintered moldings was 13.6 g/ml (the same as the theoreticaldensity). In parallel experiments, lower sintering temperatures wereused, but these did not lead to a satisfactory sinter density.

The sintered moldings had a Vickers hardness 0.5 of 950, but furthermechanical properties were not determined. The parts were ground andpolished. The microstructure was virtually pore-free, but giant graingrowth was observed; crystallite sizes up to more than 100 micrometersin diameter had formed, which were visible to the naked eye andsubstantially impaired the visual appearance of the polished parts.

A comparison of Example 1 with Comparative example 1 shows that thenovel sintered moldings not only have excellent mechanical propertiesbut also have advantages in applications where the visual impression ofthe sintered molding is decisive.

We claim:
 1. A hard sintered molding having a nickel- and cobalt-free,nitrogen-containing steel as a binder and a hard material as the hardphase, the binder being an austenitic iron alloy which contains not morethan 0.5% by weight of carbon, from 2 to 26% by weight of manganese,from 11 to 24% by weight of chromium, from 2.5 to 10% by weight ofmolybdenum, not more than 8% by weight of tungsten and from 0.55 to 1.2%by weight of nitrogen.
 2. A hard sintered molding as claimed in claim 1,the hard material being tantalum carbide, tungsten carbide, niobiumcarbide, titanium nitride and/or zirconium nitride.
 3. A hard sinteredmolding as claimed in claim 2, the hard material being tantalum carbideand/or tungsten carbide.
 4. A process of producing the sintered moldingdefined in claim 1, which comprises providing a thermoplastic injectionmolding material which comprises a thermoplastic binder material, theaustenitic iron alloy or a nitrogen-free precursor of the iron alloy inpowder form, and a hard material in powder form, the thermoplasticinjection molding material in a powder injection molding process to givea molded article, and removing the thermoplastic binder material to givea molding, and sintering the molding, and, if the thermoplasticinjection molding material contained the nitrogen-free precursor of theiron alloy, which further comprises nitriding the iron alloy before,during or after sintering the molding.
 5. A thermoplastic injectionmolding material which contains an austenitic iron alloy, which containsnot more than 0.5% by weight of carbon, from 2 to 26% by weight ofmanganese, from 11 to 24% by weight of chromium, from 2.5 to 10% byweight of molybdenum, not more than 8% by weight of tungsten and from0.55 to 1.2% by weight of nitrogen, or a nitrogen-free precursor of thisiron alloy in powder form, a hard material in powder form, and athermoplastic.